This invention relates to an electrophotographic photosensitive member sensitive to light rays (for example, electromagnetic waves such as ultraviolet rays, visible rays, infrared rays, X-rays, and gamma rays).
An electrophotographic photosensitive member has its surface, for example, positively charged by means of corona discharge. When light is irradiated on the surface of said photosensitive member with a predetermined pattern, pairs of electrons and holes are produced in that region of the photoconductive layer which has been irradiated by light, thereby neutralizing the surface charge by said electrons. Hole carriers travel through the photoconductive layer to a conductive support. On the other hand, that portion of the photosensitive member which was not irradiated by light retains a positive charge. As a result, an electrostatic latent image is formed by a positive charge. When a toner is statically adsorbed to the surface of said photosensitive member from a developer, the latent image produced in the surface of said photosensitive member is rendered visible. In this case, developing bias voltage is impressed on an area defined between the photosensitive member and developer to produce an electric field acting in an opposite direction to that in which an electric field produced by the electric charge of the surface of the photosensitive member.
The above-mentioned electrophotographic photosensitive member should have the features that the electric charge resulting from the corona discharge is sustained until development is performed after the irradiation of light rays, that is, the subject photosensitive member has a light charge-retaining capacity. Moreover, the carrier pair generated by light irradiation should be prevented from being bonded together once more and one of said pair acts to neutralize the surface charge of the photosensitive member and the other is quickly transmitted to the conductive support. Namely, the carrier should have a long life and be featured by its satisfactory traveling property.
An electrophotographic photosensitive member which should have the above-mentioned properties has hitherto been prepared from a material of amorphous chalcogenide series, for example, Se. Though able to occupy a large area, this amorphous chalcogenide material has the drawback that since the end portion of a light-absorbing region is positioned near the ultraviolet ray region involved in the visible rays, said amorphous chalcogenide has a low photosensitivity to the visible light rays and long wave light rays, and moreover, has a short life due to its low hardness.
On the other hand, amorphous silicon (as used herein, abbreviated as "a - Si") indeed has the merits that it can absorb light rays having a broad range of wavelengths so that it has a high light sensitivity over a broad range of wavelengths. Moreover, a - Si is featured by a long life due to its great hardness. In addition, a - Si does not adversely affect the human body during manufacture and can be produced in a large area at low cost. Recently, therefore, a - Si has received wide attention as a desirable material for an electrophotographic photosensitive member. Yet, a - Si still has the drawbacks that it generally has a specific resistivity as low as 10.sup.8 to 10.sup.10 .OMEGA.cm, in the dark, (hereinafter referred to as "dark resistivity"), and moreover, has a low charge retention capability.
To avoid the above-mentioned shortcomings, the conventional practice is to interpose an insulation layer of, for example, silicon nitride or silicon oxide or set up a barrier consisting of p-type or n-type a - Si between the conductive substrate and photoconductive layer of the photosensitive member, thereby to prevent carriers from being introduced from the conductive substrate into the photoconductive layer. If, in this case, a positive charge is handled, the barrier is prepared from p-type a - Si in order to allow for the passage of holes alone. If a negative charge is handled. The harrier is formed of n-type a - Si. Further, a surface layer is deposited on the photoconductive layer in order to raise the surface potential of the photosensitive member. Said surface layer is generally prepared from insulative material having a high specific resistivity.
Where, however, a thick insulative layer is applied to the surface of the photoconductive layer, the carrier transmitted from the photoconductive layer to the conductive substrate is obstructed in its travel, thus undesirably resulting in a high residual potential. If conversely a thin insulation layer is applied, a dielectric breakdown tends to occur due to the bias of development.
Further, if it is attempted to form p-type a - Si, it is necessary to dope an element belonging to Group III of the periodic table in a - Si. If it is attempted to form n-type a - Si, it is necessary to dope an element belonging to Group V of the periodic table in a - Si. However, the addition of an impurity causes noticeable strains to be produced in the a - Si layer. When, therefore, a photoconductive layer is deposited on the a - Si layer, noticeable differences appear in the strain of the respective layers. This tends to give rise to the foliation of the layers.
Where the surface layer of the photoconductive layer is prepared from insulative material, the photosensitivity of the resultant product will drop, because carriers travel at a slow speed, and the residual potential is raised. Consequently the thickness of the surface layer should generally be limited to 50 to 1000 .ANG.. However, the surface layer preferably has great thickness in order to ensure a chemical stability against any charge in the condition of the atmosphere in which the photosensitive member is applied.